[A 13-year-old boy with a mild form of cystic fibrosis and heterozygote gene mutation for Delta F508].

We report a 13-year old boy with a chronic pseudomonas bronchitis who was first diagnosed as having cystic fibrosis at this age because of an elevated sweat chloride employing pilocarpine-iontophoresis. He is heterozygote for the gene mutation Delta F508. We point out the often moderate course of illness in compound heterozygotes.